.tree-checkbox li a, .tree-checkbox li a.unchecked {
    background-image: url("check_0.png");
}

.tree-checkbox li a.undetermined {
    background-image: url("check_1.png");
}

.tree-checkbox li a.checked {
    background-image: url("check_2.png");
}

.tree-checkbox li a.clicked, .tree-checkbox li span.clicked {
    background-color: transparent;
    border-color: transparent;
    color: black;
}